The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and, more particularly, to a large capacity paper feeder for feeding a great amount of papers or recording media to the image forming apparatus.
A paper feeder with a large capacity table (LCT) is usable with an electrophotographic copier or similar image forming apparatus. Usually, the LCT is capable of sucking 1,000 or more papers thereon. Hence, the LCT is desirable when images representative of documents of the same size should be sequentially formed on papers of the same size by use of, e.g., a sorter, or when papers of particular size frequency used in daily office work should be replenished a minimum number of times. However, when images are to be formed on papers different in size from the papers stacked on the LCT, such a number of papers existing on the LCT must be entirely removed from the LCT, resulting in labor-consuming work. In order to reduce the labor, Japanese Utility Model Publication No. 61-36517 and Japanese Patent Publication No. 4-33691, for example, each proposes an arrangement for allowing a cassette loaded with papers of particular size to be mounted w the paper feeder in addition to the LCT. However, this kind of scheme requires an exclusive paper feeding mechanism for the cassette in a cassette loading section, and paper transport paths each extending from one of the paper feeding mechanisms assigned to the LCT and cassette to a photoconductive drum.
A printer, particularly a stencil printer, is different from the electrophotographic copier in that it uses a master representative of a document image. Therefore, the stencil printer is desirable when a great number of printings must be produced from a single master. It is not rare that the stencil printer produces thousands of printings with a single master or document Japanese Patent Laid-Open Publication No. 6-171819, for example, discloses a stencil printer for office use and allowing about 1,000 papers to he stacked on its tray (table). To produce thousands of printings continuously with this stencil printer, there is needed a paper feeder with an LCT similar to the LCT of the electrophotographic copier. However, even with the stencil printer, it is sometimes necessary to change the size of papers to be used Specifically, the stencil printer is not always operated to produce thousands of printings, and a great number of papers are sometimes left on the LCT.
Because replacing the papers stacked on the LCT is labor-consuming, as stated earlier, a cassette for stacking papers different in size from the papers present on the LCT is essential. This, however, is not practicable without providing a cassette loading section with an exclusive paper feeding mechanism and an exclusive transport path, and therefore without increasing the size and cost of the mechanism. Particularly, the stencil printer transports the papers at a higher speed than the electrophotographic copier. Hence, if the papers are routed through such a complicated transport path, they are apt to jam the path or to be creased In addition, it is likely that the accuracy of registration is lowered in the transport direction, and that the transport becomes defective when relatively thick papers are used.
Approaches to eliminate the above problems are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 6-271104, 6-271118, and 6-271120. A paper feeder using any one of such approaches is implemented as a large capacity paper feeder with an interruption type cassette and feeding papers by use of a single pick-up roller. More specifically, this type of paper feeder allows the interruption type cassette to be mounted to the top of a paper feed unit. The paper feeder has the paper feed unit moved up and down when papers are fed from the cassette, a cassette table or first table formed with a notch for the pick-up roller to pass therethrough when the table is moved together with the paper feed unit, and an LCT or second table movable up and down in the paper feed unit independently. The paper feed unit and LCT are each moved up and down in a single frame by a respective drive mechanism When the LCT is used, the top of the paper feed unit is brought to a position above the pick-up roller, so that papers can be fed from the LCT.
With the large capacity paper feeder having a single pick-up roller and operable with the cassette, it is necessary to promote the smooth switching between the cassette and the LCT, smooth replenishment of papers, and smooth jam processing. When the number of papers left on each of the cassette and LCT, particularly the LCT, is small, a substantial period of time is necessary for the LCT to rise from its lowered position to a paper feed position. In light of this, control capable of setting up a paper feed condition in a short period of time is desired. Japanese Patent Laid-Open Publication Nos. 6-271119 and 6-271124, for example, propose implementations for meeting the above needs. Particularly, a paper feeder taught in the above 6-271119 has a LCT/cassette select button (T/K switch button) for allowing the operator to select either a cassette feed mode or an LCT feed mode, and a LCT/cassette down key (T/K down button; tray down key). When these buttons are operated in combination, a paper feed unit loaded with a cassette or an LCT is lowered for jam processing or paper replenishment. A paper feeder disclosed in the above 6-271124 includes cassette sensing means. When a power switch is turned on, the cassette sensing means outputs a signal indicative of whether or not the cassette is present. If the cassette is absent, mode setting means sets an LCT mode. In the LCT mode, the upward or downward movement of the LCT is so controlled as to feed papers from the LCT, so that the waiting time up to the start of printing is reduced.
However, the paper feeders disclosed in the above 6-271104, 6-271118, and 6-271120 have the following problems.
(1) To feed papers from the cassette, a bulky paper feed unit having the LCT thereinside is necessary. This, coupled with the drive mechanism for moving the paper feed unit up and down in the cassette feed mode, complicates the construction and increases the overall size of the paper feeder.
(2) In the LCT mode, a cassette table positioned in the upper portion of the paper feed unit is positioned above the pick-up roller. Hence, a substantial space for accommodating the cassette table must be provided above the pick-up roller. Particularly, in the stencil printer for office use and having a master making mechanism and a printing mechanism constructed integrally with each other, as taught in the above 6-171819, various sections including a master making section and a master discharging section are each arranged at a particular position in the printer body. Hence, if the space is provided above the pick-up roller, the various sections of the printer body and, therefore, a document reading unit disposed above them must be shifted upward. As a result, the space increases the overall height of the printer. This not only increases the size of the stencil printer and that of the paper feeder, but also obstructs the easy stacking of documents on the document reading unit.
(3) The paper feeders shown and described in the above 6-271104, 6-271118 and 6-271120 need, due to their inherent constructions, a bulky cover which should be opened for mounting the cassette or for replenishing papers into t he LCT or the cassette. The bulky cover is not desirable because it needs an exclusive space to be opened and closed, because it increases the size and cost of a mold, and because it is likely to cause the operator to stack papers in an unexpected position.
The above 6-271119 and 6-271124 have the following problems.
(4) In the paper feeder taught in the above 6-271119, when the cassette is used, the up-down movement of the paper feed unit must be controlled independently of the control for moving the LCT up and down. For this purpose, the T/K switch button and T/K down button are used in combination in order to control the downward movement of the LCT. This kind of operation is extremely troublesome. In addition, an increase in the number of buttons or keys directly translates into an increase in cost.
(5) The paper feeder disclosed in the above 6-271124 successfully reduces the waiting time when papers are fed from the LCT. However, when papers should be replenished while the cassette feed mode is set up, it is necessary for the operator to remove the cassette, operate the T/K switch button in order to select the LCT feed mode, and then press the T/K down button for lowering the LCT. With a stencil printer, it often occurs that thousands of printings are produced from a single document, as stated earlier. In such a case, even if the LCT is capable of being loaded with thousands of papers (usually, limited 3,000 to 4,000 because the height of the printer body is selected to be lower than 1 meter in consideration of maneuverbility, papers must be replenished, if short, before the start of printing. Hence, it is not always desirable from the maneuverbility standpoint, including replenishment, that the papers stacked on the LCT be located it the paper feed position. Rather, it is desirable that a certain amount of papers be easily replenished within a range which does not increase the waiting time up to the start of paper feed to a noticeable degree.
In order to eliminate the above problems (1)-(3), the present invention proposes a paper feeder which in a cassette feed mode does not move a bulky paper feed Unit up or down, but has a cassette driving mechanism independent of an LCT driving mechanism. Assigning an exclusive driving mechanism to a cassette is taught in previously mentioned 61-36517 or 4-33691. Further, Japanese Patent Publication No. 61-50858, for example, shows in its FIGS. 3 and 14 a cassette driving mechanism having a shaft (123) extending throughout the side panels of a casing, and a mechanism (59) for moving a tray (stack plate) included in the cassette up and down. The mechanism 59 is located at a position substantially corresponding to the center of the cassette in order to obviate, e.g., the inclination of papers stacked on the cassette, thereby insuring the stable movement of the cassette.
However, the above cassette driving mechanism has a problem that when both the papers of the LCT and the papers of the cassette are to be fed by a single paper feeding means (pick-up roller), an elevation member included in the mechanism 59 obstructs the elevation of the papers stacked on the LCT.